In Oblivion, when a Dremora attacks they use the works Fettlekyn and Nikyn, I think refering to something it's trying chop up into bits -- so I guess they are not terms of endearment.

They also say "Kyn and Valkyn" -- I think Valkyn is Mehrune Dagon's personal guard, right? So what is Kyn?

the suffix "...kyn" appears to be common and so I'm guessing it has some meaning.

Also what does "nithing" mean?

Is there a source for this, I can't find anything through searches.

"Kyn" is the name of the species of dremora you meet in Oblivion. Any use of it with a prefix usually refers to a rank.

Like Markynaz, Valkyn, (Kynreeve is essentially a sherriff, or Shire-reeve of a castle).

"Kyn" is how they refer to themselves. Cognate with "kin".

So, to a first approximation:
"Kyn and Valkyn", "for the people and the rulers". Compare "SPQR".
"Nikyn", "not Kyn", anyone that is in the disgraceful state of not being a Dremora.
"Nithing", "nothing", similarly.

Not sure about "fettlekyn". I'll take a wild guess that it means "slave to the Kyn", one that isn't a Dremora.

Doesn't 'kyn' mean 'the people'? 'Fettle' means state, or conditions, so by crying "weak fettlekyn" they are commenting that the mortal state of being is inferior.

Fettlekyn might be their word for "mortal".

What is the source for this?

See "Varieties of Daedra".

I always thought "fettle" had something to do with food. But then, do Dremora even eat? It's likely they do to sustain their bodies and keep from the darkness of Oblivion, but for their total, continued survival it probably isn't neccessary. But like six, they seem capable of doing it for enjoyment. Which raises another question: Is "your flesh is mine" a general or grimily specific threat?

"Fettles" usually mean shackles as far as I know. They're calling opponents "slaves", probably.

Edit: the definition I looked up on the internet says fettle is "condition" -- someone being in "fine fettle", for example. So I don't know, LN. Sorry.

I believe the word you were thinking of was 'fetters', which does indeed mean shackles.

Right. Thanks, Mirander.